Instead of having each socket op conditionally compile as either the
implementation or a DIE() depending on #HAS_SOCKET

1: remove the conditional code from the ops themselves
2: only compile the ops if HAS_SOCKET is defined
3: general conditional code for the intialisation of PL_ppaddr - as appropriate
either the ops, or Perl_unimplemented_op
4: Amend Perl_unimplemented_op to generate the appropriate DIE() for socket
ops (ie not the "panic"... message)

Whilst this complicates the support code in regen/opcode.pl, it's already a
net saving of 5 lines in the C code.